Anisotropy refers to a medium with properties that depend on a direction of measurement. In one example, the speed of acoustic waves that travel through an elastically anisotropic medium will vary depending on wave propagation direction and polarization direction (e.g., direction of particle displacement by a propagating elastic wave). Heterogeneity is defined as a variation of a property with location (e.g., variation of acoustic velocities with spatial coordinates as a result of variation of clay volume with spatial coordinates).
The presence of elastic anisotropy can have significant and relevant implications. For instance, subsurface stresses in elastically anisotropic media can be very different (e.g., both in magnitude and direction) from those existing in elastically isotropic media. Subsurface stress magnitude and orientation are important pieces of information for completion design (e.g., hydraulic fracturing) in unconventional reservoirs, such as gas shales. In seismic studies, if unaccounted for, the presence of elastic anisotropy in formations can lead to errors in time-to-depth conversion, normal moveout (NMO) correction, dip moveout (DMO) correction, migration, and amplitude versus offset (AVO) analysis.